1.Field of the Invention
The present invention relates to a scanning probe microscope used for, e.g., surface measurement, surface treatment and surface processing.
2. Description of the Related Art
Scanning tunneling microscope (to be referred to as STM hereinafter) have recently been developed for observing each atom on the surface of a sample. According to the STM, an electrically conductive probe with a sharp tip is scanned over the surface of an electrically conductive sample. By measuring a tunnel current flowing between the sample and the probe during scanning, a structure of an atomic-level of the surface of the sample can be measured.
The STM is applicable to various uses, other than measurement of surface structure of a sample. For example, after the surface structure is measured, the probe is moved to a desired location on the sample surface and a pulse voltage is applied between the probe and the sample, thereby subjecting a portion just under the probe to atomic-scale fine processing. Further, by striking the probe on the sample surface, a fine recess/projection can be formed on the sample surface. That is, the STM is applicable as a processing apparatus. In addition, when gas molecules or liquid molecules are present between the probe and the sample, the molecules can be absorbed on, or separated from, the sample by an electric field generated by applied voltage. In this case, the STM is used as a surface treatment apparatus.
In the scanning probe microscope including STMs, vibration between the probe and the sample must be reduced as much as possible in order to achieve high resolution. For this purpose, the resonance frequency of the scanning probe microscope must be increased. Further, since the limit of operation speed is determined by the resonance frequency, it is necessary to increase the resonance frequency of the scanning probe microscope in order to achieve high-speed operation. As stated above, the resonance frequency of the scanning probe microscope must be increased to achieve high resolution and high-speed operation, and for this purpose the size of the driving unit of the scanning probe microscope is generally reduced. To reduce the size of the driving unit, a piezoelectric element formed of ceramics is generally used as scanning means for scanning the surface of the sample by the probe. The piezoelectric element is coupled directly to the probe or coupled directly to a sample holder, thereby reducing the size of the driving unit.
When the surface of a sample is measured by this type of scanning probe microscope at high temperatures, the following problems occur.
The piezoelectric element is arranged near the sample. When the sample is heated at high temperatures, the temperature of the piezoelectric element also rises. The piezoelectric element is generally formed of PZT ceramic material, and the Curie temperature of PZT ceramic material is in the range of 200.degree. C. to 400.degree. C. Thus, if the sample is heated at the Curie temperature or above, the temperature of the piezoelectric element is also raised up to the Curie temperature and the piezoelectric element does not operate.
In order to arrange the piezoelectric element at a distance from the probe and sample, a heat buffer may be provided between the piezoelectric element and probe or between the piezoelectric element and sample. Thereby, the piezoelectric element can operate while the sample is heated at high temperatures. In this case, however, the distance between the piezoelectric element and probe or between the piezoelectric element and sample is large, and as a result the resonance frequency of the scanning probe microscope decreases. Consequently, the scanning probe microscope cannot be operated with high resolution and at high speed with the sample heated at high temperatures.
In addition, the resonance frequency is determined not only by the distance between the piezoelectric element and sample and the distance between the piezoelectric element and probe, but also by the weight of members supported by the piezoelectric elements, i.e. a sample holder including the sample and a probe holder including the probe. In order to heat the sample at high temperatures, a heater using as heating means is required and the weight of the load supported by the piezoelectric element is increased. As a result, the resonance frequency of the scanning probe microscope decreases and the resolution and operation speed at high temperatures of the scanning probe microscope further decreases.
It is desired that a probe used in the scanning probe microscope have one atom projecting stably from a tip portion thereof. The material of the probe is generally selected from the group consisting of tungsten, platinum and a platinum/iridium compound. The reason for this is as follows. Of all elements, the melting point of tungsten is highest and most stable, and therefore tungsten is used as material of the probe. Although tungsten is widely used in a high vacuum atmosphere, the surface of a tungsten probe is easily oxidized. Thus, tungsten is not suitable for use in the air. Instead, a platinum probe whose surface is not easily oxidized is suitable for use in the air. Further, by mixing 10% to 20% of iridium in platinum, the hardness of the resultant compound is increased. Thus, a platinum/iridium compound is widely used. Furthermore, since it is important that the material of the probe is free from surface oxidation in the air, it is reported that rhenium oxide which is an oxide and has high electrical conductivity is used as material of the probe.
Even where any of the above materials is used for the probe, however, the atom at the tip of the probe is unstable. In fact, there is always a problem that the resolution of an STM image varies during STM measurement. There is no case where the resolution does not vary over several minutes. It is understood that this is due to constant movement of the atom at the probe tip.
As has been stated above, in the conventional scanning probe microscope, the temperature of the piezoelectric element rises if the sample is heated at high temperatures, and the piezoelectric element does not operate if its temperature exceeds the Curie point. Further, if the distance between the piezoelectric element and sample or between the piezoelectric element and probe is increased to prevent temperature rise of the piezoelectric element, the resonance frequency decreases and the high resolution and high-speed operation cannot be achieved.
Furthermore, in the conventional scanning probe microscope, the resolution varies constantly and high resolution is not stably obtained.
In addition, when the scanning probe microscope is applied to very fine processing, the processing is controlled only by controlling voltage across the probe and the sample. However, voltage control is not sufficient for controlling very fine processing in which atoms or molecules are transferred between the probe and sample. More excellent control means is desired.
Techniques relating to the present invention are disclosed in K. Nakamura et al., Jpn. J. Appl. Phys. Vol. 26, pp. 198-200, 1987 and S. Ikebe et al., Jpn. J. Appl. Phys. Vol. 30, pp. L405-406, March, 1991. Further, there are U.S. Pat. No. 5,148,026 and U.S. Pat. No. 5,089,740.